Reduce the "head" required by closing the vacuum breaker

[rogerbohl]

I'm helping my son manage his energy bill. One idea I have is to reduce the "head" required by a pump by closing the vacuum break (in summer) at the top of the solar panels (or moving it to a lower point in the panel plumbing network) so that the pump is aided by the siphoning within the solar panel loop. While this will reduce the pressure the pump must overcome by, say 15psi, as discussed on this thread, the gauge pressure would go negative at the top of the solar panels. How can I determine whether the negative pressure will cause flexing and/or collapse of the panels, that could result in increased internal resistance or, worse, physical damage to the panels?

I may or may not be able to identify the manufacturer of the panels, as they were installed before my son bought the house.

 

[Isaac-1]

Re: Solar Panels and Vacuum Breakers

I don't see this working as you imagine

 

[JasonLion]

Welcome to TFP!

You don't want the pressure in the panels to actually go negative, since the small water tubes will collapse and pinch off the water flow. However, your approach can still work to some extent. A vacuum breaker requires some positive pressure to close, while the panels will work fine at neutral pressure. By lowering the vacuum breaker you can eliminate the pressure that is required just to make sure the vacuum breaker closes.

 

[jblizzle]

Re: Reduce the "head" required by closing the vacuum breaker

But I still do not understand how this could possibly save energy costs?

If you want to save money on electricity, you have to move less water.

 

[rogerbohl]

Jason,
Thanks for the caution and modest encouragement. I'll experiment, and also try to ID and seek info from the panel manufacturer. Energy would be saved if the vacuum break could be closed entirely, because the gauge (relative to atmospheric) pressure at the bottom of a siphon is 0, whereas at the bottom of a 15 foot pipe with the top open to the atmosphere is about half of atmospheric pressure (7psi). (Atmospheric pressure = 30" of mercury, or 34' of water.) Moving the same quantity of water (per second) against a the greater pressure requires more work per second = more energy. I have not calculated the energy savings because I have not ascertained the flow rate -- I'll be installing a flow meter in a week or so. I'll also have to dust off my fluid mechanics training -- my last class was 52 years ago!

Equivalent mechanical example: Consider the force required to lift a garage door with and without counterbalance (the equivalent of a siphon). With perfect counterbalance, one only has to overcome friction. Ditto a seesaw with and without the counterbalance of a person on the other end.

 

[Isaac-1]

Again I think your looking at it wrong, by moving the anti siphon vale to the bottom of the panel you can save the slight over pressure needed to secure it being close if it were at the top of the panel, say 1 -2 psi, but your not going to see a significant negative pressure at the top of the panel while the pump is operating as you expect. Do tell us a bit more about this solar array though, how many panels are there, what size, do they have 1.5 or 2 inch headers, is the plumbing 1.5 or 2 inch, etc.

 

[mas985]

Are you attempting to run the panels on low speed of a two speed pump or lower speeds of a VS pump?

Most single speed pumps should provide more than enough pressure on a two story install to keep the VRV closed. If it isn't then there could be an issue with the install.

 

[jblizzle]

Based on reading numerous threads here, I do not think the power usage for constant speed electric motors work like you think they do. For the same pump/motor running at constant RPM, if you increase the head loss and thus pressure, you will reduced the flow rate and the electricity consumption.

Once the solar panels are primed, you have gravity helping out and there is not much head loss due to the vertical distance. It sounds like you are thinking that if you were able to create a vacuum in the panels (not a good idea) that it would help the pump prime the panels, but that is such a short period of time I do not think it would have any impact on power use.

Although now Mark is here, so he can straighten me out if I misspeak

 

[JasonLion]

The potential savings comes from using a variable speed pump at a slightly lower speed. The pump still needs to lift the water to the top of the roof, but it no longer needs to produce the additional pressure required to close a vacuum relief valve at the very top. That allows one to run at a low speed. The pump is more efficient at lower speeds, thus energy savings. That is the only situation with energy savings I can think of, and the savings are fairly small, but still enough to notice.

 

[rogerbohl]

I posted a reply(ies?), but it(they) may not have gone to the new thread. Being a newbie here, I don't know how to find or fix.

 

[JasonLion]

I posted a reply(ies?), but it(they) may not have gone to the new thread. Being a newbie here, I don't know how to find or fix.

When I check the forum logs, I only see three posts from you, all in this one topic.